1. Field
The invention relates to a stripping device for use with a cutting tool with a cutting element, in particular a punch, for machining a workpiece, in particular a curved metal sheet, at least one fastening element for fastening it to the cutting tool, a spring-elastic element arranged outside the workpiece contact region, a stripping element which comes into contact with the workpiece and surrounds the cutting element, and at least one guide element guiding the stripping element being provided.
2. Description of the Related Art
Stripping devices are known in conjunction with various types of cutting tools (DE 196 05 113 A1, DE 40 35 938 A1, DE 42 35 972 A1 and WO 99/67038 A1). A stripping device of this type is required for enabling, in particular in the case of punches or other cutting elements, the machined workpiece, in particular metal sheet, to be stripped off from the cutting element, in particular punch. During the cutting process, in particular punching process, a front surface of the stripper is placed against the surface of the workpiece, deflects inward somewhat during the penetration of the workpiece by the punch and, when the cutting tool is pulled back out of the workpiece, springs out again, thus ensuring that the, for example, punch will be pulled out of the workpiece.
Various models of strippers are available commercially. Most of them have a fastening plate by means of which they can be fastened to the cutting tool, in particular a punch fastening plate. The stripper body is composed, for example, as a rubber spring of a hard plastic, the front surface of which is formed in accordance with the contour of the workpiece. The shaping can be undertaken here by trimming. The rubber spring surrounds the punch on all sides. In most cases, the shape of the front surface of the stripper is not symmetrical, since the workpiece to be machined generally has an irregular shaping.
U.S. Pat. No. 2,168,377 discloses a stripping device for use with a punch for machining a flat, planar metal sheet, in which an outer element is fastened to a specially configured retaining plate of a cutting tool via screws and bolts. The outer element is provided on its inside with a longitudinal opening into which a stripping element, and in it the punch, are fitted. A spring-elastic element in the form of a helical spring is fitted between the stripping element, the outer element and the punch. The stripping element has an essentially straight section and a protruding section, which can be supported on a projection within the longitudinal opening of the outer element or is secured thereon in order not to be pushed inadvertently out of the element.
U.S. Pat. No. 1,723,935 discloses a similar construction of a stripping device as the above publication. In the same manner as said publication, U.S. Pat. No. 1,723,935 also uses a helical spring which is arranged within an outer guide sleeve between punch, stripping element and a special fastening device for fastening it to the cutting tool. The outer guide sleeve is screwed onto a fastening piece which is fastened to a further fastening piece which is connected via a flange to the cutting tool via a screw connection.
U.S. Pat. No. 4,993,295 discloses a stripping device for use with a punch for machining a planar metal sheet, in which, as in U.S. Pat. No. 1,723,935, guide surfaces between an outer guide sleeve and a stripping element are relatively short, which means that, in the event of higher loads, the stripping element may tilt within the guide sleeve. As spring-elastic element, various disk springs are provided which are layered on one another within the guide sleeve in such a manner that the curved surfaces in each case are directed toward one another. A punch is arranged within the spring-elastic element.
The stripper also has the task of keeping the workpiece in the desired shape during the machining process. This is particularly important if punchings are to be undertaken in the region of metal-sheet edges, since deformations may easily occur there because of the punching process. However, the stripper is not intended to automatically deform the workpiece, but merely to keep the latter in the desired, premanufactured shape. If a rubber spring stripper which completely surrounds a punch and has an irregular front shaping facing the metal sheet is provided, this proves problematic if the stripper, after a number of punching processes, rotates around the punch. The shaping of the surface of the stripper then does not correspond to the shaping of the surface of the metal sheet which is to be punched, for which reason problems of quality and complaints may occur in this case.
For this purpose, DE 812 498 discloses a stripping device for a punch with a helical spring which is arranged between a stripper plate and a punch head. The helical spring surrounds the region of the punch. Three strips are provided which are fastened between the punch head and stripper plate and maintain a distance between these two elements. The fastening takes place via screws and elongated holes, so that the distance between stripper plate and punch head can be adjusted. By contrast, a rotation of the stripper plate is scarcely possible because of the strips.
FR 1 456 310 discloses a stripping device which, in one embodiment, comprises a helical spring and, in another, comprises an elastic element which is fitted between two fixed plates. In the case of the second embodiment, a screw bolt is arranged between the two fixed plates, in a similar manner as provided by the strips in DE 812 498 for stably connecting the two plates. The screw bolt can also prevent the plates from rotating in relation to each other.
However, these publications do not disclose any possibility of allowing matching to the particular shaping of deformed metal sheets. In all of the publications only straight metal sheets are ever punched. However, it is required in particular in the automobile industry to provide stripping devices which match the particular shapings of deformed metal sheets or can be matched to them without any problem, essentially do not leave any traces on the punched metal sheets and have a long service life, i.e. of such stable design that they withstand a large number of strokes, in particular more than one million strokes without any maintenance. In addition, the stripping device is to be designed in such a manner that a simple and rapid changing and interchanging of stripping devices can be undertaken. This is not possible with the devices of the prior art that are fastened to the cutting tool in a complicated manner.
In order to prevent rotation, Dayton Progress GmbH has also disclosed a spring-mounted steel stripper. A means of securing against rotation for the punch is formed by the punch being constricted in cross section in a subregion and being flattened in rectangular form. A section of the stripper, which section is fastened to the steel stripper by means of screws, engages in this region.
This solution proves to be disadvantageous because of its susceptibility to failure due to limited stability in the region of the small fastening screws and risk of fracture of the constricted punch. Since the individual parts of a stripper have to be matched precisely to one another and a complicated mounting is frequently required, in the case of alternatives manufactured manually average prices of 1800 euros per piece arise. In contrast to this, the pure rubber strippers, as described further above, cost approx. 100 euros per piece. However, these have the disadvantage, in addition to the disadvantages already mentioned above, that, when a metal sheet is punched from the inside, only small piece numbers for use with only certain shapes are possible. The steel stripper also has the disadvantage that the screws retaining the engaging section on the steel stripper are very small and frequently do not permanently withstand the forces which occur, particularly since said screws are loaded transversely. The durability of a rubber stripper, as described above, is approx. 80 000 strokes, meaning that correct stripping is no longer ensured or possible and the manufacturing reliability is therefore impaired.